For the pyrolysis or incineration of carbonaceous waste materials in fluidized beds, it is advantageous to employ an inert particulate solid as the medium for transferring heat to the carbonaceous solids. For this purpose, quartz sand has excellent heat capacity and heat transfer properties. See U.S. Pat. Nos. 3,772,999; 3,736,111; 3,776,150; and 3,853,498.
Although both pyrolysis and incineration of carbonaceous wastes can be carried out in fluidized bed reactors containing quartz sand as the heat transfer medium, as indicated by the above-cited patents, the operating conditions of these two processes are quite different. Pyrolysis is carried out under endothermic, reducing conditions, while exothermic, oxidizing conditions are used for incineration. Heretofore, the system was believed to be practical for commercial use only as applied to incineration and not for pyrolysis of carbonaceous wastes to produce a fuel gas. AWT Systems, Inc. of Wilmington, Del., has marketed incineration apparatus under the name "Fluidhearth", which is believed to be similar to that described in U.S. Pat. Nos. 3,772,999 and 3,776,150, assigned to AWT Systems, Inc.
U.S. Pat. No. 3,853,498 describes a system for converting municipal wastes into high energy fuel gas by endothermic pyrolysis. The system uses two sand-containing fluidized beds. In one bed, the sand is heated by an exothermic combustion reaction, and in the other fluidized bed the heated sand is used in the endothermic pyrolysis reaction. Means are provided for transferring the sand between the two beds, and the char produced in the pyrolysis reaction is recycled with the sand for removal in the combustion reaction. There is no known commercial use of such a system.
There has been a major unsolved problem with respect to the use of sand in fluidized beds for pyrolytic conversion of carbonaceous materials to fuel gas. Although the beds are operated at temperatures well below the initial melting temperature of quartz sand (about 1900.degree. F.), after a few hours of operation the sand bed contains agglomerates or "clinkers". Such bed agglomeration or slagging seriously interferes with the fluidization of the bed, necessitating shut-down of the process for removal of the clinkers and replacement of sand. Very little is known about the nature of this agglomeration problem, and prior to the present invention no one has disclosed an adequate solution.
Robert S. Burton studied the conversion of solid wastes to fuel gas in quartz sand fluidized beds. This research is reported in his report submitted for an M.S. degree at the West Virginia University, Morgantown, W.Va., entitled "Fluid Bed Gasification of Solid Waste Materials", dated 1972. The following discussion appears at pages 167-168 of the Burton report:
"Another problem encountered in the experimental equipment which must be considered in the final process design is the problem of slagging. During the experimental portion of the investigation, it was found that when MSW [municipal solid waste] was fed in a hot reducing atmosphere (1300.degree.-1500.degree. F. at 0.0-0.5 percent oxygen) slag formed. This slag accumulated over the life of the experiment and finally `choked` the bed ceasing fluidization. The pilot plant facility had no provisions for ash draw off. Also the MSW fed to the fluid bed was merely buried and no attempt was made to remove any ash or inert materials (metals, ceramics, etc.) It appears that the slagging problem centers around the ash in the MSW. In the reducing atmosphere, the ash softening temperature is approximately 300.degree. F. lower than in an oxidizing atmosphere, which brings it into the operating range. Removing the ash and inerts is a step in the right direction but it probably will not completely solve the problem. At best it will only delay `choking`." PA1 "Based on the development work performed, the present concept is to use sand in both the pyrolysis and the combustion reactor beds as a heat carrier. Sand does not melt at the proposed operating temperatures, but various alkaline impurities in the refuse may convert the silica to lower melting silicates. It may be desirable to evaluate other materials, such as limestone, dolomite, and petroleum coke."
At the request of West Virginia University, the Stanford Research Institute, Menlo Park, Calif., conducted a further study of the pyrolysis process described in the cited Burton thesis. This study included a consideration of the slagging or clinker problem. The results of the study are contained in a report entitled "Pyrolysis of Solid Wastes: A Technical and Economic Assessment", SRI Project ECC-1886, prepared for West Virginia University, September, 1972. Page 45 of the report discusses the problem, stating: "A practical means for preventing the agglomeration of the granular bed material [quartz sand] and of providing for its withdrawal from the reactor are needed". The report then discusses possible solutions relating to the selection of the heat carrier, and/or the continuous removal and replacement of bed material. With reference to the heat transfer medium, the reports states (page 45):
The above-cited SRI Project report in Appendix C presents an analysis of the clinkers formed in the quartz sand bed. Analysis of samples of the aggregated sand disclosed the presence of two substances which might act as cementing materials. These were bottle glass and various forms of iron oxide. In this connection, the report states (page C-3): "Bottle glass has a softening point above 1100.degree. F. and would become a viscous liquid at the temperature of the reactor."
The foregoing represents the state of the art prior to the present invention, which for the first time makes it practical to employ on a commercial basis a quartz sand fluidized bed for pyrolysis of carbonaceous material to fuel gas. This represents a highly important development because it provides a means for disposing of waste materials, such as municipal garbage, sewage sludge, used tires, etc., and at the same time providing a new and relatively inexpensive source of fuel gas, thereby helping to alleviate the present energy crisis.